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u <br />• <br />r~ <br />i <br />Relocation of Water Mains and Underground Stormwater Drainage. Installed in 1998. Refer to <br />"1997f98 Landslide Corrective Measures Report" for a detailed description of the relocations. Unreported <br />1n 1997/98 report, the stormwater drainage line from above the F-Seam Portal to MB-1 pond was replaced <br />with a large diameter HDPE pipeline. <br />Horizontal Dewatering Trenches. Installation completed in December 1998. A major groundwater <br />source is near the surface, at the head of the landslide. A series of 10 horizontal dewatering trenches were <br />constructed along the head of the slope to intercept near surface groundwater. The trenches are <br />constructed up to 30 feet deep. A gravel drainage material is used that will filter sand size material from <br />the pressurized sand layers. The drains are aligned parallel to the slope so they do not shear as the <br />landslide continues to move. Figures Hl and H4 show the location of these trenches in plan and cross <br />section, and Appendix B of the 1997/98 report shows a detail of the trench design. <br />H3.2 On-going and Potential Corrective Measures <br />Planned for construction during 1999 and 2000 are ongoing efforts to reduce groundwater levels within the <br />slide. th addition, potential corrective measures aze ready to be implemented if conditions warrant. These <br />ongoing measures include the following work. <br />F-Seam Sumps. The F-Seam mine workings collect measurable amounts of colluvial groundwater, which <br />infiltrates from the surface through cracks and fissures into the mine. Due [o the 3 to 5 degree NE dip in <br />the coal seams, the lowest points in the F-Seam workings aze near the portal entry ways. Here water <br />collects in make-shift sumps and ditches, is removed by pumps, and piped to sedimentation ponds. The <br />sumps are poorly constructed and some of [he water is lost through fractures in the rock or out of the <br />entryway itself. It is probable that some of this water finds its way to the head of the slide. The proposed <br />solution is to install directional drilled horizontal drains that intercept water in the sump and fractured <br />bedrock, and cant' it out to the sedimentation ponds, thus preventing infiltration into the landslide. <br />Engineering analysis and design is in progress with construction anticipated in the Fall and Winter of 1999 <br />or Winter of 2000. Figure H2 shows a preliminary layout of the proposed plan. <br />Barr Engineering Company <br />PCDOCS1206220/CAK <br />July 31, 7999 <br />12 Appendix H <br />1999 Addendum to <br />'1997/98 Landslide Corrective Measures Report <br />